Mitochondrial DNA is a small piece of DNA that's located in structures outside the nucleus, these structures called the mitochondria. The mitochondria are actually ancient bacteria that entered the early cells when they were still just single living cells about three billion years ago and have formed a symbiotic relationship in which the mitochondria live as a conolinear bacteria inside our cells. Now because they are bacteria they have their own information system, which is DNA and they convert that information into structures in the form of RNA and then proteins.